U.S. patent application number 11/184601 was filed with the patent office on 2006-05-18 for beam-steering and beam-forming for wideband mimo/miso systems.
This patent application is currently assigned to QUALCOMM Incorporated. Invention is credited to Steven J. Howard, John W. Ketchum, Murali Paravath Menon, Mark Wallace, Jay Rod Walton.
Application Number | 20060104381 11/184601 |
Document ID | / |
Family ID | 31976026 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060104381 |
Kind Code |
A1 |
Menon; Murali Paravath ; et
al. |
May 18, 2006 |
Beam-steering and beam-forming for wideband MIMO/MISO systems
Abstract
Techniques to perform beam-steering and beam-forming to transmit
data on a single eigenmode in a wideband multiple-input channel. In
one method, a steering vector is obtained for each of a number of
subbands. Depending on how the steering vectors are defined,
beam-steering or beam-forming can be achieved for each subband. The
total transmit power is allocated to the subbands based on a
particular power allocation scheme (e.g., full channel inversion,
selective channel inversion, water-filling, or uniform). A scaling
value is then obtained for each subband based on its allocated
transmit power. Data to be transmitted is coded and modulated to
provide modulation symbols. The modulation symbols to be
transmitted on each subband are scaled with the subband's scaling
value and further preconditioned with the subband's steering
vector. A stream of preconditioned symbols is then formed for each
transmit antenna.
Inventors: |
Menon; Murali Paravath;
(Waltham, MA) ; Ketchum; John W.; (Harvard,
MA) ; Wallace; Mark; (Bedford, MA) ; Walton;
Jay Rod; (Carlisle, MA) ; Howard; Steven J.;
(Harvard, MA) |
Correspondence
Address: |
QUALCOMM, INC
5775 MOREHOUSE DR.
SAN DIEGO
CA
92121
US
|
Assignee: |
QUALCOMM Incorporated
|
Family ID: |
31976026 |
Appl. No.: |
11/184601 |
Filed: |
July 18, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10228393 |
Aug 27, 2002 |
6940917 |
|
|
11184601 |
Jul 18, 2005 |
|
|
|
Current U.S.
Class: |
375/267 ;
375/260 |
Current CPC
Class: |
H04L 2025/03414
20130101; H04L 25/0204 20130101; H04B 7/043 20130101; H04B 7/0417
20130101; H04L 25/0248 20130101; H04W 52/42 20130101; H04L
2025/03426 20130101; H04L 5/006 20130101; H04L 5/0025 20130101;
H04B 7/0634 20130101; H04L 5/0096 20130101; H04L 25/03343 20130101;
H04L 25/0246 20130101; H04L 5/0053 20130101; H04B 7/0617 20130101;
H04L 5/0044 20130101; H01Q 3/26 20130101; H04L 2025/03802 20130101;
H04L 5/0085 20130101 |
Class at
Publication: |
375/267 ;
375/260 |
International
Class: |
H04B 7/02 20060101
H04B007/02; H04K 1/10 20060101 H04K001/10 |
Claims
1. A method for processing data for transmission via a wideband
multiple-input channel, comprising: obtaining a steering vector for
each of a plurality of subbands, wherein each steering vector
includes a plurality of elements for a plurality of transmit
antennas; and preconditioning modulation symbols to be transmitted
on each subband with the steering vector for the subband.
2. The method of claim 1, wherein each steering vector achieves
beam-steering for the associated subband.
3. The method of claim 1, wherein the elements of each steering
vector have equal amplitude.
4. The method of claim 1, wherein each steering vector achieves
beam-forming for the associated subband.
5. The method of claim 1, further comprising: obtaining a plurality
of scaling values for the plurality of subbands; and scaling the
modulation symbols for each subband with the scaling value for the
subband.
6. The method of claim 5, wherein the scaling values for the
subbands are determined based on gains for the subbands provided by
the steering vectors.
7. The method of claim 5, wherein the scaling values for the
subbands are determined based on transmit powers allocated to the
subbands.
8. The method of claim 7, wherein the transmit powers are allocated
to the subbands based on full channel inversion.
9. The method of claim 7, wherein the transmit powers are allocated
to the subbands based on selective channel inversion.
10. The method of claim 7, wherein the transmit powers are
allocated to the subbands based on uniform power allocation.
11. The method of claim 7, wherein the transmit powers are
allocated to the subbands based on water-filling power
allocation.
12. The method of claim 1, wherein the multiple-input channel is a
multiple-input multiple-output (MIMO) channel.
13. The method of claim 12, wherein the steering vector for each
subband is derived based on an eigenvector corresponding to a
principal eigenmode.
14. The method of claim 1, wherein the multiple-input channel is a
multiple-input single-output (MISO) channel.
15. The method of claim 1, further comprising: coding and
modulating data based on a common coding and modulation scheme to
provide the modulation symbols.
16. The method of claim 1, further comprising: forming a stream of
preconditioned symbols for each transmit antenna; and processing
each stream of preconditioned symbols to provide a modulated signal
for transmission from a respective transmit antenna.
17. The method of claim 1, wherein the wideband system implements
orthogonal frequency division multiplexing (OFDM), and wherein the
plurality of subbands correspond to orthogonal subbands provided by
OFDM.
18. In a multiple-input communication system that implements
orthogonal frequency division multiplexing (OFDM), a method for
processing data for transmission via a multiple-input channel
comprising: obtaining a steering vector for each of a plurality of
subbands, wherein each steering vector includes a plurality of
elements for a plurality of transmit antennas; obtaining a
plurality of scaling values for the plurality of subbands; scaling
modulation symbols to be transmitted on each subband with the
scaling value for the subband; preconditioning the scaled
modulation symbols for each subband with the steering vector for
the subband; and forming a stream of preconditioned symbols for
each transmit antenna.
19. The method of claim 18, wherein each steering vector achieves
beam-steering for the associated subband.
20. The method of claim 18, wherein the scaling values for the
subbands are determined based on selective channel inversion.
Description
CLAIM OF PRIORITY UNDER 35 U.S.C. .sctn.120
[0001] The present Application for Patent is a continuation and
claims priority to patent application Ser. No. 10/228,393 entitled
"BEAM-STEERING AND BEAM-FORMING FOR WIDEBAND MIMO/MISO SYSTEMS"
filed Aug. 27, 2002, pending, and assigned to the assignee hereof
and hereby expressly incorporated by reference herein.
BACKGROUND
[0002] 1. Field
[0003] The present invention relates generally to data
communication, and more specifically to techniques for performing
beam-steering and beam-forming for wideband MIMO/MISO systems.
[0004] 2. Background
[0005] A multiple-input multiple-output (MIMO) communication system
employs multiple (N.sub.T) transmit antennas and multiple (N.sub.R)
receive antennas for data transmission. A MIMO channel formed by
the N.sub.T transmit and N.sub.R receive antennas may be decomposed
into N.sub.S independent channels, with N.sub.S.ltoreq.min
{N.sub.T, N.sub.R}. Each of the N.sub.S independent channels is
also referred to as a spatial subchannel or eigenmode of the MIMO
channel.
[0006] A multiple-input single-output (MISO) communication system
employs multiple (N.sub.T) transmit antennas and a single receive
antenna for data transmission. A MISO channel formed by the N.sub.T
transmit and single receive antenna includes a single spatial
subchannel or eigenmode. However, the multiple transmit antennas
may be used to provide transmit diversity or to perform
beam-forming or beam-steering for the data transmission.
[0007] For a wideband system, orthogonal frequency division
multiplexing (OFDM) may be used to effectively partition the
overall system bandwidth into a number of (N.sub.F) orthogonal
subbands, which are also referred to as frequency bins or
subchannels. With OFDM, each subband is associated with a
respective subcarrier upon which data may be modulated. For a
MIMO/MISO system that utilizes OFDM (i.e., a MIMO/MISO-OFDM
system), each subband of each spatial subchannel may be viewed as
an independent transmission channel.
[0008] The spatial subchannel(s) of a wideband MIMO/MISO system may
encounter different channel conditions due to various factors such
as fading and multipath. Each spatial subchannel may experience
frequency selective fading, which is characterized by different
channel gains at different frequencies of the overall system
bandwidth. This may then result in different signal-to-noise ratios
(SNRs) at different frequencies of each spatial subchannel.
Moreover, the channel conditions may deteriorate to a level where
most of the spatial subchannels are highly degraded. In these
situations, improved performance may be achieved by using only the
best spatial subchannel for data transmission.
[0009] There is therefore a need in the art for techniques to
process data for transmission on a single spatial subchannel when
warranted by the channel conditions.
SUMMARY
[0010] Techniques are provided herein to transmit data on a single
spatial subchannel (or eigenmode) in a wideband multiple-input
system, which may be a MIMO or MISO system (e.g., a MIMO-OFDM or
MISO-OFDM system). These techniques may be used to provide improved
performance under adverse channel conditions.
[0011] Data transmission on a single eigenmode (typically the best
or principal eigenmode for a MIMO system) may be achieved using
beam-steering or beam-forming. For a wideband MIMO/MISO system, the
beam-steering or beam-forming is performed for each subband that is
selected for use for data transmission based on a steering vector
obtained for that subband. The beam-steering or beam-forming may
also be performed in conjunction with a particular power allocation
scheme that allocates the total transmit power to the subbands.
[0012] In an embodiment, a method is provided to process data for
transmission via a single eigenmode of a multiple-input channel
(e.g., a MIMO or MISO channel). In accordance with the method, a
steering vector is obtained for each of a number of subbands. Each
steering vector includes N.sub.T elements for N.sub.T transmit
antennas. Depending on how the steering vectors are defined,
beam-steering or beam-forming can be achieved for each subband.
[0013] The total transmit power is allocated to the subbands based
on a particular power allocation scheme (e.g., full channel
inversion, selective channel inversion, water-filling, or uniform,
all of which are described below). A scaling value is then obtained
for each subband based on the transmit power allocated to the
subband.
[0014] Data to be transmitted is coded and modulated based on one
or more coding and modulation schemes to provide modulation
symbols. The modulation symbols to be transmitted on each subband
are then scaled with the subband's scaling value, and the scaled
modulation symbols are further preconditioned with the subband's
steering vector. A stream of preconditioned symbols is then formed
for each transmit antenna, and this stream is further processed to
generate a modulated signal suitable for transmission from a
respective transmit antenna.
[0015] Various aspects and embodiments of the invention are
described in further detail below. The invention further provides
methods, program codes, digital signal processors, transmitter
units, receiver units, and other apparatuses and elements that
implement various aspects, embodiments, and features of the
invention, as described in further detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The features, nature, and advantages of the present
invention will become more apparent from the detailed description
set forth below when taken in conjunction with the drawings in
which like reference characters identify correspondingly throughout
and wherein:
[0017] FIG. 1 graphically illustrates the results of eigenvalue
decomposition for a number of subbands in a MIMO-OFDM system;
[0018] FIG. 2 is a block diagram of a transmitter system and a
receiver system in the MIMO-OFDM system;
[0019] FIG. 3 is a block diagram of a transmitter unit within the
transmitter system;
[0020] FIG. 4 is a block diagram of a signal scaling unit, a
beam-steering unit, and a multiplexer within the transmitter unit;
and
[0021] FIG. 5 is a flow diagram for processing data for
transmission on a single eigenmode of a multiple-input channel
using beam-steering or beam-forming.
DETAILED DESCRIPTION
[0022] The beam-steering and beam-forming techniques described
herein may be used in various wideband MIMO/MISO communication
systems. For clarity, these techniques are described specifically
for a MIMO-OFDM system that effectively divides the overall system
bandwidth into N.sub.F orthogonal subbands.
[0023] The model for the MIMO-OFDM system may be expressed as:
y(k)=H(k)x(k)+n(k), for k.di-elect cons.{1, . . . , N.sub.F} Eq (1)
where y(k) is a vector with N.sub.R entries, {y.sub.i(k)} for
i.di-elect cons.{1, . . . , N.sub.R}, for the symbols received via
the N.sub.R receive antennas for the k-th subband (i.e., the
"received" vector); [0024] x(k) is a vector with N.sub.T entries,
{x.sub.j(k)} for j.di-elect cons.{1, . . . , N.sub.T}, for the
symbols transmitted from the N.sub.T transmit antennas for the k-th
subband (i.e., the "transmit" vector); [0025] H(k) is an
(N.sub.R.times.N.sub.T) channel response matrix with entries,
{h.sub.ij(k)} for i.di-elect cons.{1, . . . , N.sub.R} and
j.di-elect cons.{1, . . . , N.sub.T}, which are the complex gains
from the N.sub.T transmit antennas to the N.sub.R receive antennas
for the k-th subband; and [0026] n(k) is additive white Gaussian
noise (AWGN) for the k-th subband, with zero mean and a covariance
matrix of .LAMBDA..sub.n=.sigma..sup.2I, where I is the identity
matrix and .sigma..sup.2 is the noise variance.
[0027] For simplicity, each subband is assumed to be frequency
non-selective (i.e., with a flat frequency response across the
entire subband). In this case, the channel response h.sub.ij(k) for
each transmission channel can be represented by a single complex
value, and the elements of the channel response matrix H(k) are
scalars. Also for simplicity, the noise variance is assumed to be
constant across all transmission channels. For a time division
duplexed (TDD) system, the forward and reverse links share the same
system bandwidth and each subband may be assumed to be reciprocal.
That is, if H(k) represents the channel response matrix from
antenna array A to antenna array B, then a reciprocal channel
implies that the coupling from array B to array A is given by
H.sup.H(k).
[0028] The channel response matrix H(k) for each subband may be
"diagonalized" to obtain the N.sub.S independent channels for that
subband. This can be achieved by performing eigenvalue
decomposition on the correlation matrix of H(k), which is
R(k)=H.sup.H(k)H(k), where H.sup.H(k) denotes the conjugate
transpose of H(k). The eigenvalue decomposition of the correlation
matrix R(k) may be expressed as: R(k)=E(k)D(k)E.sup.H(k), for
k.di-elect cons.{1, . . . , N.sub.F} Eq (2) where E(k) is an
(N.sub.T.times.N.sub.T) unitary matrix whose columns are the
eigenvectors of R(k); and [0029] D(k) is an (N.sub.T.times.N.sub.T)
diagonal matrix with entries on the diagonal corresponding to the
eigenvalues of R(k) . A unitary matrix is denoted by the property
M.sup.HM=I.
[0030] The eigenvalue decomposition may also be performed using
singular value decomposition, as is known in the art.
[0031] The diagonal matrix D(k) for each subband contains
non-negative real values along the diagonal and zeros everywhere
else. These diagonal entries are referred to as the eigenvalues of
R(k) and are related to the complex gains for the independent
channels (or eigenmodes) of the MIMO channel for the k-th subband.
Since the number of independent channels is N.sub.S.ltoreq.min
{N.sub.T, N.sub.R} for a MIMO system with N.sub.T transmit and
N.sub.R receive antennas, there are N.sub.S non-zero eigenvalues of
R(k). The eigenvalues of R(k) are denoted as {.lamda..sub.i(k)},
for i={1, . . . , N.sub.S} and k={1, . . . , N.sub.F}.
[0032] For the MIMO-OFDM system, the eigenvalue decomposition may
be performed independently for the channel response matrix H(k) for
each subband to determine the N.sub.S eigenmodes for that subband.
The N.sub.S eigenvalues for each diagonal matrix D(k), for
k.di-elect cons.{1, . . . , N.sub.F}, may be ordered such that
{.lamda..sub.1(k).gtoreq..lamda..sub.2(k).gtoreq. . . .
.gtoreq..lamda..sub.N.sub.S(k))}, where .lamda..sub.1(k) is the
largest eigenvalue and .lamda..sub.N.sub.S(k) is the smallest
eigenvalue for the k-th subband.
[0033] FIG. 1 graphically illustrates the results of the eigenvalue
decomposition for the N.sub.F subbands in the MIMO-OFDM system. The
set of diagonal matrices, D(k) for k={1, . . . , N.sub.F}, is shown
arranged in order along an axis 110 that represents the frequency
dimension. The eigenvalues, {.lamda..sub.i(k)} for i={1, . . . ,
N.sub.S}, of each matrix D(k) are located along the diagonal of the
matrix. Axis 112 may thus be viewed as representing the spatial
dimension. Eigenmode i for all subbands (or simply, eigenmode i) is
associated with a set of elements, {.lamda..sub.i(k)} for k={1, . .
. , N.sub.F}, which is indicative of the frequency response across
the N.sub.F subbands for that eigenmode. The set of elements
{.lamda..sub.i(k)} for each eigenmode is shown by the shaded boxes
along a dashed line 114. Each shaded box in FIG. 1 represents a
transmission channel. For each eigenmode that experiences frequency
selective fading, the elements {.lamda..sub.i(k)} for that
eigenmode may be different for different values of k.
[0034] If the eigenvalues in each diagonal matrix D(k) are sorted
in descending order, then eigenmode 1 (which is also referred to as
the principal eigenmode) would include the largest eigenvalue in
each matrix, and eigenmode N.sub.S would include the smallest
eigenvalue in each matrix.
[0035] Under adverse channel conditions, most of the eigenmodes may
be highly degraded. In these situations, improved performance may
be achieved by using only the best eigenmode (i.e., the principal
eigenmode) for data transmission.
[0036] The model for a MISO-OFDM system may be expressed as:
y(k)=h(k)x(k)+n(k), for k.di-elect cons.{1, . . . , N.sub.F}, where
y(k) denotes the symbol received on the k-th subband; [0037] x(k)
is a vector with N.sub.T entries for the symbols transmitted from
the N.sub.T transmit antennas for the k-th subband; [0038] h(k) is
a (1.times.N.sub.T) channel response vector with entries,
{h.sub.j(k)} for j.di-elect cons.{1, . . . , N.sub.T}, which are
the complex gains from the N.sub.T transmit antennas to the single
receive antenna for the k-th subband; and [0039] n(k) is additive
white Gaussian noise (AWGN) for the k-th subband.
[0040] For MIMO and MISO systems, data transmission on a single
eigenmode may be achieved using beam-steering or beam-forming, both
of which are described below.
1. Beam-Forming
[0041] The beam-forming technique transmits data on a single (e.g.,
principal) eigenmode by preconditioning the modulation symbols with
the eigenvector for this eigenmode. For the MIMO-OFDM system, the
beam-forming is performed for each subband using the eigenvector
obtained for that subband.
[0042] In equation (2), the unitary matrix E(k) contains N.sub.T
columns for N.sub.T eigenvectors, i.e., E(k)=[e.sub.1(k) e.sub.2(k)
. . . e.sub.N.sub.T(k)]. The eigenvectors are also referred to as
steering vectors. Each eigenvector is associated with a respective
eigenmode and eigenvalue of the diagonal matrix D(k) (i.e.,
eigenvector e.sub.i(k) is associated with eigenvalue
.lamda..sub.i(k) for subband k). When the eigenvalues of D(k) are
sorted in descending order as described above, the eigenvectors of
E(k) are also rearranged in the corresponding order. After the
sorting/rearrangement, eigenvector e.sub.1(k) corresponds to the
largest eigenvalue .lamda..sub.1(k) and is the eigenvector for the
principal eigenmode for the k-th subband. This eigenvector
e.sub.1(k) includes N.sub.T elements for the N.sub.T transmit
antennas and can be expressed as:
e.sub.1(k)=[e.sub.1,1(k)e.sub.1,2(k) . . .
e.sub.1,N.sub.T(k)].sup.T, for k.di-elect cons.{1, . . . , N.sub.F}
Eq (3) where ".sup.T" denotes the transpose.
[0043] The preconditioning at the transmitter to achieve
beam-forming for each subband may be expressed as: x(k)= {square
root over (P(k))}e.sub.1(k)s(k), for k.di-elect cons.{1, . . . ,
N.sub.F} Eq (4) where s(k) is the modulation symbol to be
transmitted on the k-th subband; [0044] {square root over (P(k))}
is a scaling value derived based on the transmit power P(k)
allocated to the k-th subband; and [0045] x(k) is the transmit
vector with N.sub.T preconditioned symbols for the k-th subband. As
shown in equation (4), the beam-forming technique generates one
transmit vector x(k) for each subband based on the eigenvector
e.sub.1(k) for the principal eigenmode. Since the elements of the
eigenvector e.sub.1(k) may have different magnitudes, the elements
of the transmit vector x(k) may also have different magnitudes.
[0046] For each transmit antenna i, the N.sub.F preconditioned
symbols to be transmitted on the N.sub.F subbands in symbol period
n are multiplexed together into a (per-antenna transmit) vector
x.sub.i(n), which may be expressed as:
x.sub.i(n)=[e.sub.1,i(1){tilde over (s)}(1)e.sub.1,i(2){tilde over
(s)}(2) . . . e.sub.1,i(N.sub.F){tilde over (s)}(N.sub.F)].sup.T,
for i.di-elect cons.{1, . . . , N.sub.T}, where {tilde over (s)}(k)
is a scaled modulation symbol and given as {tilde over (s)}(k)=
{square root over (P(k))}s(k).
[0047] For the MISO-OFDM system, the beam-forming is also performed
for each subband using the steering vector obtained for that
subband. If the channel decomposition is performed on the channel
response vector h(k), the result will be one eigenmode (i.e., one
non-zero value for the matrix D(k)) and one steering vector. This
steering vector will be equal to h*(k). The beam-forming for MISO
may be performed as shown in equation (4).
2. Beam-Steering
[0048] The beam-steering technique transmits data on the principal
eigenmode by preconditioning the modulation symbols with a
"normalized" steering vector for this eigenmode. The beam-steering
is also performed for each subband for the MIMO-OFDM system.
[0049] As noted above, the elements of each eigenvector e.sub.1(k),
for k.di-elect cons.{1, . . . N.sub.F}, for the principal eigenmode
may have different magnitudes. Consequently, the per-antenna
transmit vectors x.sub.i(n), for i.di-elect cons.{1, . . . ,
N.sub.T}, may have different magnitudes. If the transmit power for
each transmit antenna is limited (e.g., because of limitations of
the power amplifiers), then the beam-forming technique may not
fully use the total power available for each antenna.
[0050] The beam-steering technique uses only the phase information
from the eigenvectors e.sub.1(k), for k.di-elect cons.{1, . . . ,
N.sub.F}, and normalizes each transmit steering vector such that
all N.sub.T elements have equal magnitudes. The normalized steering
vector {tilde over (e)}(k) for the k-th subband may be expressed
as: e(k)=[Ae.sup.j.sup..theta..sup.1.sup.(k)
Ae.sup.j.sup..theta..sup.2.sup.(k) . . .
Ae.sup.j.sup..theta..sup.N.sup.T.sup.(k)].sup.T, where A is a
constant (e.g., A=1); and [0051] .theta..sub.i(k) is the phase for
the k-th subband of the i-th transmit antenna, which is given as:
.theta. i .function. ( k ) = .angle. .times. .times. e 1 , i
.function. ( k ) = tan - 1 .function. ( Im .times. { e 1 , i
.function. ( k ) } Re .times. { e 1 , i .function. ( k ) } ) . Eq
.times. .times. ( 5 .times. b ) ##EQU1## As shown in equation (5b),
the phase of each element in the vector {tilde over (e)}(k) is
obtained from the corresponding element of the eigenvector
e.sub.1(k) (i.e., .theta..sub.i(k) is obtained from
e.sub.1,i(k)).
[0052] The preconditioning at the transmitter to achieve
beam-steering for each subband may be expressed as: x(k)= {square
root over (P(k))}{tilde over (e)}(k)s(k), for k.di-elect cons.{1, .
. . , N.sub.F} Eq (6) As shown in equations (5a) and (5b), the
elements of the normalized steering vector {tilde over (e)}(k) for
each subband have equal magnitude but possibly different phases.
The beam-steering technique generates one transmit vector x(k) for
each subband, with the elements of x(k) having the same magnitude
but possibly different phases.
[0053] As described above, for each transmit antenna i, the N.sub.F
preconditioned symbols to be transmitted on the N.sub.F subbands in
symbol period n are multiplexed together into a per-antenna
transmit vector x.sub.i(n). Since each transmit vector x.sub.i(n),
for i.di-elect cons.{1, . . . , N.sub.T}, includes the same set of
scaled modulation symbols (but possibly with different phases), the
total available transmit power for each antenna may be fully
used.
[0054] At the receiver, to obtain an estimate of the modulation
symbol s(k), the received vector y(k) for each subband may be
pre-multiplied (or "conditioned") with either {tilde over
(e)}.sup.H(k)H.sup.H(k) (if beam-steering was performed) or
e.sub.1.sup.H(k)H.sup.H (k) (if beam-forming was performed). If
beam-steering was performed, then the conditioning to obtain the
symbol estimate s(k) may be expressed as: s ^ .function. ( k ) = e
_ ~ H .function. ( k ) .times. H _ H .function. ( k ) .times. y _
.function. ( k ) = P .function. ( k ) .times. e _ ~ H .function. (
k ) .times. H _ H .function. ( k ) .times. H _ .function. ( k )
.times. e _ ~ .function. ( k ) .times. s .function. ( k ) + e _ ~ H
.function. ( k ) .times. H _ H .function. ( k ) .times. n _
.function. ( k ) = P .function. ( k ) .times. D .function. ( k )
.times. s .function. ( k ) + n ^ .function. ( k ) , Eq .times.
.times. ( 7 ) ##EQU2## where D(k) is the beam-steering gain for the
k-th subband, which can be expressed as D(k)={tilde over
(e)}.sup.H(k)H.sup.H(k)H(k){tilde over (e)}(k), and Eq (8) [0055]
{circumflex over (n)}(k) is AWGN with zero mean and a noise
variance of .sigma..sup.2D(k).
[0056] The received signal-to-noise ratio (SNR) for the k-th
subband with beam-steering may be expressed as: .gamma. bs
.function. ( k ) = P .function. ( k ) .times. D .function. ( k )
.sigma. 2 , for .times. .times. k .di-elect cons. { 1 , .times. , N
F } . Eq .times. .times. ( 9 ) ##EQU3##
[0057] The spectral efficiency for the k-th subband may be computed
based on a continuous, monotonically increasing logarithmic
function for capacity, as follows:
C.sub.bs(k)=log.sub.2(1+.gamma..sub.bs(k)), for k.di-elect cons.{1,
. . . , N.sub.F} Eq (10) The spectral efficiency is given in units
of bit/second per Hertz (bps/Hz). The mean (average) spectral
efficiency for the N.sub.F subbands of the MIMO-OFDM system may
then be expressed as: C _ bs = k = 1 N F .times. C bs .function. (
k ) N F . Eq .times. .times. ( 11 ) ##EQU4##
[0058] Similar computations may be performed for the beam-forming
technique.
[0059] For the MISO-OFDM system, the beam-steering is also
performed for each subband using a normalized steering vector
obtained for that subband. The normalized steering vector for MISO
may be obtained in similar manner as that described above for the
normalized steering vector {tilde over (e)}(k) for the principal
eigenmode (i.e., using the phase of the steering vector). The
beam-steering for MISO may be performed as shown in equation
(6).
3. Power Allocation for the Subbands
[0060] If the total transmit power for all N.sub.T transmit
antennas is limited to a particular value P.sub.total, then the
beam-forming technique may provide better results than the
beam-steering technique. This is because the total transmit power
may be more optimally distributed across the N.sub.T transmit
antennas based on the eigenvectors e.sub.1(k) for the principal
eigenmode. However, if the transmit power available for each
transmit antenna is limited (e.g., to P.sub.total/N.sub.T), then
the beam-steering technique would likely achieve better results
than the beam-forming technique. This is because the beam-steering
technique can more fully use all of the power available for each
transmit antenna.
[0061] In any case, the total transmit-power P.sub.total may be
distributed across the N.sub.T transmit antennas and the N.sub.F
subbands using various power allocation schemes. These schemes
include (1) full channel inversion, (2) selective channel
inversion, (3) uniform, and (4) "water-filling" or "water-pouring"
power allocation schemes. For clarity, each of these schemes is
specifically described below for the beam-steering technique.
4. Full Channel Inversion
[0062] If the same amount of transmit power is used for each
subband, then beam-steering can result in different received SNRs
for the N.sub.F subbands. To maximize spectral efficiency, a
different coding and modulation scheme may then be used for each
subband depending on the SNR achieved for the subband. However,
coding and modulating individually for each subband can
significantly increase the complexity of both the transmitter and
receiver. On the other hand, if the same coding and modulation
scheme is used for all subbands, then there may be significant
variation in the error rates for the N.sub.F subbands, depending on
the variation in the received SNRs.
[0063] Full channel inversion may be used to effectively "invert"
the subbands such that the received SNRs for all subbands are
approximately equal. The power allocation may be performed under
the constraint that the total power allocated to all subbands for
each transmit antenna is limited to P.sub.ant=P.sub.total/N.sub.T.
For full channel inversion, the amount of transmit power P(k) to
allocate to each subband may be expressed as: P .function. ( k ) =
.alpha. k .times. P total N T .times. N F , for .times. .times.
.times. k .di-elect cons. { 1 , .times. , N F } , Eq .times.
.times. ( 12 ) ##EQU5## where .alpha..sub.k is a scaling factor
used for the full channel inversion power allocation. The scaling
factor for the k-th subband may be expressed as: .alpha. k = b D
.function. ( k ) , Eq .times. .times. ( 13 ) ##EQU6## where b is a
normalization factor that may be expressed as: b = 1 k = 1 N F
.times. D .function. ( k ) - 1 . Eq .times. .times. ( 14 )
##EQU7##
[0064] As shown in equations (12) and (13), the total transmit
power P.sub.total is distributed unevenly across the N.sub.F
subbands based on the scaling factors .alpha..sub.k, for k.di-elect
cons.{1, . . . , N.sub.F}, which are inversely related to the
beam-steering gains D(k). The scaling factors .alpha..sub.k ensure
that the received SNRs for all subbands are approximately equal.
The received signal power P.sub.rx(k) for each subband may be given
as: P rx .function. ( k ) = P .function. ( k ) .times. D .function.
( k ) 2 = .alpha. k .times. P total .times. D .function. ( k ) 2 N
T .times. N F = bP total .times. D .function. ( k ) N T .times. N F
, for .times. .times. k .di-elect cons. { 1 , .times. , N F } . Eq
.times. .times. ( 15 ) ##EQU8## The noise power is given by
.sigma..sup.2D(k). The signal-to-noise ratio .gamma.(k) for subband
k is then given by: .gamma. .function. ( k ) = P .function. ( k )
.times. D .function. ( k ) 2 .sigma. 2 .times. D .function. ( k ) =
P .function. ( k ) .times. D .function. ( k ) .sigma. 2 = .alpha. k
.times. P total .times. D .function. ( k ) N T .times. N F .times.
.sigma. 2 = bP total N T .times. N F .times. .sigma. 2 . Eq .times.
.times. ( 16 ) ##EQU9## The total received signal power P.sub.rx
may then be given as: P rx = k = 1 N F .times. P .function. ( k )
.times. D .function. ( k ) 2 = bP total N T .times. N F .times. k =
1 N F .times. D .function. ( k ) . ##EQU10##
[0065] The total transmit power P.sub.total is allocated to the
subbands such that they achieve equal received SNRs (i.e., the
received SNR for each subband is not a function of k), as shown in
equation (16). This then enables the use of a common coding and
modulation scheme for all subbands while satisfying the per-antenna
power constraint.
[0066] To achieve approximately equal received SNRs for all N.sub.F
subbands, the full channel inversion scheme allocates more transmit
power to poorer subbands with low gains. Because the per-antenna
power is constrained to P.sub.total/N.sub.T, the better subbands
with higher gains are allocated less transmit power. This can
result in a reduction in the overall spectral efficiency of the
system. However, the full channel inversion may simplify the
processing at the receiver since the overall channel is effectively
flat and equalization of the received signal may not be
required.
5. Selective Channel Inversion
[0067] The selective channel inversion scheme distributes the total
transmit power P.sub.total such that the subbands selected for use
achieve approximately equal received SNRs. This may be performed by
first selecting all or only a subset of the N.sub.F subbands for
use for data transmission. The channel selection may result in the
elimination of poor subbands with low SNRs that fall below a
particular threshold. This threshold may be selected to maximize
spectral efficiency, as described below. The total transmit power
P.sub.total is then distributed across only the selected subbands
and such that their received SNRs are approximately equal.
[0068] The scaling factors {tilde over (.alpha.)}.sub.k used for
power allocation by the selective channel inversion scheme may be
expressed as: .alpha. ~ k = { b ~ D .function. ( k ) - 1 , if
.times. .times. D .function. ( k ) > .times. .rho. .times.
.times. L avg 0 , otherwise , Eq .times. .times. ( 17 ) ##EQU11##
where .rho. is a value used to set the threshold, L.sub.avg is the
average gain, and {tilde over (b)} is a normalization factor. The
normalization factor {tilde over (b)} is similar to b in equation
(14) but is computed over only the selected subbands, and may be
expressed as: b ~ = 1 D .function. ( k ) .gtoreq. .rho. .times.
.times. L avg .times. D .function. ( k ) - 1 . Eq .times. .times. (
18 ) ##EQU12## The average gain L.sub.avg may be computed as: L avg
= k = 1 N F .times. D .function. ( k ) N F . Eq .times. .times. (
19 ) ##EQU13##
[0069] As shown in equation (17), a given subband is selected for
use if its beam steering gain is greater than or equal to the
threshold (i.e., |D(k)|.gtoreq..rho.L.sub.avg). Since no transmit
power is allocated to poor subbands with gains below the threshold,
higher spectral efficiency may be attained. For the subbands
selected for use, the total transmit power P.sub.total is
distributed to these subbands based on their scaling factors {tilde
over (.alpha.)}.sub.k, similar to that shown in equation (15), such
that the received signal power for each selected subband is given
as {tilde over (b)}P.sub.totalD(k)/N.sub.TN.sub.F and all selected
subbands have approximately equal received SNR.
[0070] The threshold used to select subbands may be set based on
various criteria. The threshold that maximizes spectral efficiency
may be determined as follows. Initially, the gains D(k) for all
N.sub.F subbands are ranked and placed in descending order in a
list G(l), for l.di-elect cons.{1, . . . , N.sub.F}, such that
G(1)=max{D(k)} and G(N.sub.F)=min{D(k)}. A sequence B(l) is then
defined as follows: B .function. ( l ) = ( i = l l .times. ( G
.function. ( i ) - 1 ) ) - 1 , for .times. .times. l .di-elect
cons. { 1 , .times. , N F } . Eq .times. .times. ( 20 ) ##EQU14##
B(l) is the list of {tilde over (b)} if the best l subbands are
used.
[0071] The received SNR on all the selected subbands, which results
when the l best subbands are selected for use, is given as: .gamma.
^ .function. ( l ) = B .function. ( l ) .times. P total .sigma. 2
.times. N T . Eq .times. .times. ( 21 ) ##EQU15## For equation
(21), the total transmit power P.sub.total is allocated among the l
best subbands such that they achieve equal received SNRs.
[0072] If the l best subbands are selected for use, then the total
spectral efficiency for these subbands is given as:
C(l)=llog.sub.2(1+{circumflex over (.gamma.)}(l)) Eq (22)
[0073] The spectral efficiency C(l) may be computed for each value
of l, for l.di-elect cons.{1, . . . , N.sub.F}, and stored in an
array. After all N.sub.F values of C(l) have been computed for the
N.sub.F possible combinations of selected subbands, the array of
spectral efficiencies is traversed and the largest value of C(l) is
determined. The value of l, l.sub.max, corresponding to the largest
C(l) is then the number of subbands that results in the maximum
spectral efficiency for the channel conditions being evaluated.
[0074] The value .rho. may then be computed as: .rho. = G
.function. ( l max ) L avg , Eq .times. .times. ( 23 ) ##EQU16##
where L.sub.avg is determined as shown in equation (19). The
threshold .rho.L.sub.avg can thus be set equal to D(l.sub.max),
which is the gain of the worst subband in the group of subbands
that maximizes spectral efficiency. The threshold used for channel
selection may also be set based on some other criterion.
[0075] The received SNRs for all selected subbands can be made
approximately equal by distributing the total transmit power
P.sub.total non-uniformly across these subbands. The equal received
SNRs would then allow for the use of a single data rate and a
common coding and modulation scheme for all selected subbands,
which would greatly reduce complexity for both the transmitter and
receiver.
[0076] The full and selective channel inversion schemes are
described in further detail in U.S. patent application Ser. No.
09/860,274, filed May 17, 2001, Ser. No. 09/881,610, filed Jun. 14,
2001, and Ser. No. 09/892,379, filed Jun. 26, 2001, all three
entitled "Method and Apparatus for Processing Data for Transmission
in a Multi-Channel Communication System Using Selective Channel
Inversion," assigned to the assignee of the present application and
incorporated herein by reference.
6. Water-Filling
[0077] The water-filling scheme may be used to optimally distribute
the total transmit power across the subbands such that the overall
spectral efficiency is maximized, under the constraint that the
total transmit power is limited to P.sub.total. The water-filling
scheme distributes power to the N.sub.F subbands such that the
subbands with increasingly higher gains receive increasingly
greater fractions of the total transmit power. The transmit power
allocated to a given subband is determined by the subband's
received SNR, which is dependent on the subband's gain, as shown in
equation (9) for the beam-steering technique. The water-filling
scheme may allocate zero transmit power to subbands with
sufficiently poor received SNRs.
[0078] The procedure for performing water-filling is known in the
art and not described herein. One reference that describes
water-filling is "Information Theory and Reliable Communication,"
by Robert G. Gallager, John Wiley and Sons, 1968, which is
incorporated herein by reference. The result of the water-filling
is a specific transmit power allocation P.sub.w(k) for each of the
N.sub.F subbands. The water-filling power allocation is performed
such that the following condition is satisfied: P total = k = 1 N F
.times. P w .function. ( k ) . Eq .times. .times. ( 24 )
##EQU17##
[0079] Based on the allocated transmit powers of P.sub.w(k) for
k={1, . . . , N.sub.F}, where P.sub.w(k) may be zero for one or
more subbands, the received SNR for each subband may be expressed
as: .gamma. w .function. ( k ) = P w .function. ( k ) .times. D
.function. ( k ) .sigma. 2 , for .times. .times. k .di-elect cons.
{ 1 , .times. , N F } . Eq .times. .times. ( 25 ) ##EQU18## The
spectral efficiency C for each subband may then be computed as
shown in equation (10), and the average spectral efficiency for all
N.sub.F subbands may be computed as shown in equation (11).
[0080] The water-filling power allocation typically results in
different received SNRs for the subbands that have been allocated
non-zero transmit powers. Different coding and modulation schemes
may then be used for the selected subbands based on their received
SNRs.
7. Uniform Power Allocation
[0081] The uniform power allocation scheme distributes the total
transmit power P.sub.total uniformly across all N.sub.F subbands.
The transmit power P.sub.u(k) allocated to each subband may be
expressed as: P u .function. ( k ) = P total N T .times. N F , for
.times. .times. k .di-elect cons. { 1 , .times. , N F } . Eq
.times. .times. ( 26 ) ##EQU19##
[0082] The uniform power allocation may also result in different
received SNRs for the N.sub.F subbands. Different coding and
modulation schemes may then be used for these subbands based on
their received SNRs. If the MIMO system has a large diversity
order, then the full and selective channel inversion schemes offer
little advantage over the uniform power scheme. If the MIMO system
has a large diversity order, then the N.sub.F largest eigenvalues
for the N.sub.F subbands are not likely to vary widely. In that
case, the performance of the fall and selective channel inversion
schemes would be similar to that of the uniform power scheme.
[0083] The total transmit power may also be allocated to the
subbands based on some other power allocation schemes, and this is
within the scope of the invention.
[0084] Simulations were performed for (1) the beam-steering
technique with three different power allocation schemes (full
channel inversion, selective channel inversion, and uniform) and
(2) the beam-forming technique with uniform power allocation. When
the transmit power available for each transmit antenna is limited
(e.g., to P.sub.total/N.sub.T), the beam-steering technique
provides approximately 2.5 dB improvement in performance over the
beam-forming technique. This significant improvement can be
attributed to the fact that all of the available power is used by
the beam-steering technique, which is not the case with the
beam-forming technique. At a sufficiently low received SNR (which
is -1 dB for the specific system configuration used in the
simulations), the beam-steering technique can provide improved
performance over a technique that transmits data using all of the
eigenmodes and allocates the total transmit power uniformly across
these eigenmodes. This is because at sufficiently low received
SNRs, only a few eigenmodes are "active", and better performance
may be achieved by allocating the total transmit power to the best
eigenmode. For the beam-steering technique, selective channel
inversion performs better than full channel inversion at low
received SNRs and when the estimates of the MIMO channel are noisy.
The simulations suggest that, at low received SNRs, beam steering
with selective channel inversion is a better choice for use than
other MIMO transmission schemes
8. System
[0085] FIG. 2 is a block diagram of an embodiment of a transmitter
system 210 and a receiver system 250 in a MIMO-OFDM system 200.
[0086] At transmitter system 210, traffic data (i.e., information
bits) from a data source 212 is provided to a transmit (TX) data
processor 214, which codes, interleaves, and modulates the data to
provide modulation symbols. A TX spatial processor 220 further
processes the modulation symbols to provide preconditioned symbols,
which are then multiplexed with pilot symbols and provided to
N.sub.T OFDM modulators (MOD) 222a through 222t, one modulator for
each transmit antenna. Each OFDM modulator 222 processes a
respective stream of preconditioned symbols to generate a modulated
signal, which is then transmitted from a respective antenna
224.
[0087] At receiver system 250, the modulated signals transmitted
from the N.sub.T antennas 224a through 224t are received by N.sub.R
antennas 252a through 252r. The received signal from each antenna
252 is provided to a respective OFDM demodulator (DEMOD) 254. Each
OFDM demodulator 254 conditions (e.g., filters, amplifies, and
frequency downconverts) the received signal, digitizes the
conditioned signal to provide samples, and further processes the
samples to provide a stream of received symbols. An RX spatial
processor 260 then processes the N.sub.R received symbol streams to
provide recovered symbols, which are estimates of the modulation
symbols transmitted by the transmitter system.
[0088] The processing for the reverse path from the receiver system
to the transmitter system may be similar to, or different from, the
processing for the forward path. The reverse path may be used to
send back channel state information (CSI) from the receiver system
to the transmitter system. The CSI is used at the transmitter
system to (1) select the proper data rate(s) and coding and
modulation scheme(s) to use for data transmission, (2) perform
beam-steering or beam-forming, and (3) allocate the total transmit
power to the subbands. The CSI may be provided in various forms.
For example, to perform beam-steering, the CSI may include N.sub.T
phases for the N.sub.T transmit antennas for each subband selected
for use.
[0089] Controllers 230 and 270 direct the operation at the
transmitter and receiver systems, respectively. Memories 232 and
272 provide storage for program codes and data used by controllers
230 and 270, respectively.
[0090] The block diagram of the transmitter and receiver systems in
a MISO-OFDM system would be similar to that shown in FIG. 2.
However, the receiver system would include only one receive antenna
and no RX spatial processor 260.
[0091] FIG. 3 is a block diagram of a transmitter unit 300, which
is an embodiment of the transmitter portion of transmitter system
210 in FIG. 2.
[0092] Within TX data processor 214, an encoder 312 receives and
codes the traffic data (i.e., the information bits) in accordance
with one or more coding schemes to provide coded bits. A channel
interleaver 314 then interleaves the coded bits based on one or
more interleaving schemes to provide time, spatial, and/or
frequency diversity. A symbol mapping element 316 then maps the
interleaved data in accordance with one or more modulation schemes
(e.g., QPSK, M-PSK, M-QAM, and so on) to provide modulation
symbols.
[0093] The coding and modulation for the subbands may be performed
in various manners. If the received SNRs for the subbands are
approximately equal at the receiver system (e.g., with full or
selective channel inversion), then a common coding and modulation
scheme may be used for all subbands used for data transmission. If
the received SNRs are different, then a separate coding and
modulation scheme may be used for each subband (or each group of
subbands with approximately equal received SNRs). Convolutional,
trellis, and Turbo coding may be used to code the data.
[0094] Within TX spatial processor 220, estimates of the impulse
response of the MIMO channel are provided to a fast Fourier
transform (FFT) unit 322 as a sequence of matrices of time-domain
samples, H(n). FFT unit 322 then performs an FFT on each set of
N.sub.F matrices H(n) to provide a corresponding set of N.sub.F
estimated channel frequency response matrices, H(k) for k.di-elect
cons.{1, . . . , N.sub.F}.
[0095] A unit 324 then performs eigenvalue decomposition on each
matrix H(k) to provide the unitary matrix E(k) and the diagonal
matrix D(k), as described above. A set of gains D(k) is then
computed based on the matrices H(k) and the steering vectors, which
may be (k) or e.sub.1(k), for k.di-elect cons.{1, . . . , N.sub.F}.
The gains D(k) are provided to a power allocation unit 330 and the
steering vectors are provided to a beam-steering/forming unit
350.
[0096] Power allocation unit 330 distributes the total transmit
power P.sub.total to the subbands using any one of the power
allocation schemes described above. This results in power
allocations of P(k), for k.di-elect cons.{1, . . . , N.sub.F}, for
the N.sub.F subbands, where P(k) may be zero for one or more
subbands. Power allocation unit 330 then provides scaling values
{square root over (P(k))} for the subbands to a signal scaling unit
340.
[0097] The block diagram of the transmitter unit in a MISO-OFDM
system would be similar to that shown in FIG. 3. However, the
steering vector for each subband is derived based on a channel
response vector h(k) instead of the channel response matrix
H(k).
[0098] FIG. 4 is a block diagram of an embodiment of a signal
scaling unit 340a, a beam-steering unit 350a, and a multiplexer
360a within transmitter unit 300, which are designed to perform
beam-steering. Within signal scaling unit 340a, the modulation
symbols s(k) are demultiplexed by a demultiplexer 440 into (up to)
N.sub.F substreams, one substream for each subband to be used for
data transmission. Each symbol substream s.sub.k is provided to a
respective multiplier 442.
[0099] Each multiplier 442 performs signal scaling for an
associated subband based on the scaling value {square root over
(P(k))} provided for that subband. In particular, each multiplier
442 scales each modulation symbol in its substream with its scaling
value {square root over (P(k))} to provide a corresponding scaled
modulation symbol. The signal scaling for each modulation symbol
may be expressed as: {tilde over (s)}.sub.k=s.sub.k {square root
over (P(k))}. The scaling value {square root over (P(k))} for each
multiplier 442 is determined by the transmit power P(k) allocated
to the associated subband. Each substream of scaled modulation
symbols {tilde over (s)}.sub.k is then provided to a respective
beam-steering unit 450.
[0100] Each beam-steering unit 450 performs beam-steering for an
associated subband and also receives the normalized steering vector
{tilde over (e)}(k) for that subband. Within each unit 450, the
scaled modulation symbols {tilde over (s)}.sub.k are provided to
N.sub.T multipliers 452a through 452t, one multiplier for each
transmit antenna. Each multiplier 452 also receives a respective
element {tilde over (e)}.sub.i(k) of the normalized steering vector
{tilde over (e)}(k), multiplies each scaled modulation symbol in
the substream with the element {tilde over (e)}.sub.i(k), and
provides a preconditioned symbol x.sub.i(k) to a combiner 460 for
the transmit antenna associated with that multiplier. The
preconditioning performed by beam-steering unit 450k for the k-th
subband may be expressed as: x.sub.i(k)={tilde over
(e)}.sub.i(k){tilde over (s)}.sub.k, for i.di-elect cons.{1, . . .
, N.sub.T}. Each beam-steering unit 450 provides N.sub.T
preconditioned symbols, x.sub.i(k) for i.di-elect cons.{1, . . . ,
N.sub.T}, to N.sub.T combiners 460a through 460t for the N.sub.T
transmit antennas.
[0101] The signal scaling and preconditioning may also be combined
or performed in a different order than that described above.
[0102] Each combiner 460 receives up to N.sub.F preconditioned
symbols, x.sub.i(k) for k.di-elect cons.{1, . . . , N.sub.F}, from
up to N.sub.F beam-steering units 450 for the up to N.sub.F
subbands used for data transmission. Each combiner 460 may also
multiplex pilot symbols with the preconditioned symbols in one or
more subbands using time division multiplexing, coding division
multiplexing, and/or frequency division multiplexing. The pilot
symbols may be used at the receiver to estimate the MIMO channel.
Each combiner 460 provides a stream of preconditioned symbols to a
respective OFDM modulator 222.
[0103] Within each OFDM modulator 222, an IFFT unit 472 receives
the stream of preconditioned symbols and forms a preconditioned
symbol vector x.sub.i(n) for each symbol period. Each such vector
has N.sub.F elements for the N.sub.F subbands, and includes
preconditioned symbols for the selected subbands and zeros for the
unselected subbands (i.e., x.sub.i(n)=[x.sub.i(1) x.sub.i(2) . . .
x.sub.i(N.sub.F)]. IFFT unit 472 then performs an inverse FFT on
each vector to obtain a corresponding time-domain representation,
which is referred to as an OFDM symbol. For each OFDM symbol, a
cyclic prefix generator 474 repeats a portion of the OFDM symbol to
form a corresponding transmission symbol. The cyclic prefix ensures
that the transmission symbol retains its orthogonal properties in
the presence of multipath delay spread. A transmitter (TMTR) 476
then converts the transmission symbols into one or more analog
signals and further conditions (e.g., amplifies, filters, and
frequency upconverts) the analog signals to generate a modulated
signal that is then transmitted from the associated antenna
224.
[0104] FIG. 5 is a flow diagram of an embodiment of a process 500
for transmitting data on a single eigenmode of a multiple-input
channel using beam-steering or beam-forming. The multiple-input
channel may be a MIMO channel in a MIMO system or a MISO channel in
a MISO system. Initially, a steering vector is obtained for each of
the N.sub.F subbands (step 512). The steering vector for each
subband may be the eigenvector e.sub.1(k) for the eigenmode of that
subband (for beam-forming) or the normalized steering vector {tilde
over (e)}(k) derived based on the eigenvector e.sub.1(k) (for
beam-steering). For the MIMO system, the eigenvectors for the
subbands may be obtained by performing eigenvalue decomposition on
the matrices H(k), for k.di-elect cons.{1, . . . , N.sub.F}, as
described above. For the MISO system, there is only one eigenmode
and one steering vector for each subband. Each steering vector
includes N.sub.T elements for the N.sub.T transmit antennas. The
gain D(k) for each subband provided by its steering vector is then
determined (e.g., as shown in equation (8) for beam-steering) (step
514).
[0105] The total transmit power P.sub.total is allocated to the
subbands using any one of the power allocation schemes described
above (e.g., full channel inversion, selective channel inversion,
uniform, or water-filling) (step 516). The gains for the subbands
may be used to perform the power allocation. All or only a subset
of the N.sub.F subbands may be selected for use for data
transmission by the power allocation. A scaling value {square root
over (P(k))} is then obtained for each selected subband based on
its allocated power (step 518).
[0106] Data to be transmitted is coded and modulated based on one
or more coding and modulation schemes to obtain modulation symbols
(step 520). A common coding and modulation scheme may be used if
the received SNRs for the subbands are approximately equal. In
general, the particular coding and modulation scheme to use for
each subband is dependent on the received SNR achieved by that
subband.
[0107] The modulation symbols to be transmitted on each subband are
then scaled with the subband's scaling value (step 522). The scaled
modulation symbols for each subband are then preconditioned with
the subband's steering vector (step 524). The preconditioning
achieves beam-steering or beam-forming for the subband, depending
on whether {tilde over (e)}(k) or e.sub.1(k) is used as the
steering vector. For each subband selected for use, one vector of
N.sub.T preconditioned symbols is generated for each scaled
modulation symbol, and these N.sub.T preconditioned symbols are to
be transmitted on that subband from the N.sub.T transmit
antennas.
[0108] A stream of preconditioned symbols is then formed for each
transmit antenna by multiplexing the outputs of the preconditioning
for the selected subbands (step 526). Each preconditioned symbol
stream is further processed (e.g., OFDM modulated) to provide a
modulated signal for transmission from a respective transmit
antenna (step 528).
[0109] For clarity, specific embodiments have been described above.
Variations to these embodiments and other embodiments may also be
derived based on the teachings described herein. For example, a set
of subbands may be selected for use for data transmission based on
one or more criteria, independent of the scheme used to allocate
transmit power to the subbands. As another example, the gains D(k)
and steering vectors may be derived by the receiver system and
provided to the transmitter system as part of the CSI. The
processing for MIMO and MIMO-OFDM systems is described in further
detail in U.S. patent application Ser. No. 09/993,087, entitled
"Multiple-Access Multiple-Input Multiple-Output (MIMO)
Communication System," filed Nov. 6, 2001, assigned to the assignee
of the present application and incorporated herein by
reference.
[0110] For clarity, the techniques for performing beam-steering and
beam-forming have been described specifically for a MIMO-OFDM
system. These techniques may also be used for a MIMO system that
does not employ OFDM. The processing to achieve beam-steering or
beam-forming for each subband may be performed as described above.
However, the processing by modulators 222 would be dependent on the
particular modulation/transmission scheme selected for use.
[0111] The techniques described herein may be implemented by
various means. For example, these techniques may be implemented in
hardware, software, or a combination thereof. For a hardware
implementation, the elements used to implement any one or a
combination of the techniques (e.g., TX spatial processor 220) may
be implemented within one or more application specific integrated
circuits (ASICs), digital signal processors (DSPs), digital signal
processing devices (DSPDs), programmable logic devices (PLDs),
field programmable gate arrays (FPGAs), processors, controllers,
micro-controllers, microprocessors, other electronic units designed
to perform the functions described herein, or a combination
thereof.
[0112] For a software implementation, the techniques described
herein may be implemented with modules (e.g., procedures,
functions, and so on) that perform the functions described herein.
The software codes may be stored in a memory unit (e.g., memory
unit 232 in FIG. 2) and executed by a processor (e.g., controller
230). The memory unit may be implemented within the processor or
external to the processor, in which case it can be communicatively
coupled to the processor via various means as is known in the
art.
[0113] Headings are included herein for reference and to aid in
locating certain sections. These headings are not intended to limit
the scope of the concepts described therein under, and these
concepts may have applicability in other sections throughout the
entire specification.
[0114] The previous description of the disclosed embodiments is
provided to enable any person skilled in the art to make or use the
present invention. Various modifications to these embodiments will
be readily apparent to those skilled in the art, and the generic
principles defined herein may be applied to other embodiments
without departing from the spirit or scope of the invention. Thus,
the present invention is not intended to be limited to the
embodiments shown herein but is to be accorded the widest scope
consistent with the principles and novel features disclosed
herein.
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